1. Field of the Invention
This invention relates to a computer system and a method for changing the operation rate of a system bus.
2. Description of the Related Art
Recently, with the development of semiconductor technology, microprocessors, memories and peripheral control Large Scale Integrated Circuits ("LSI") can be supplied at extremely low cost. Computer systems of relatively high performance can be constructed by adequately combining the LSIs via system buses. To enhance the operation rate of such computer systems, the operation rate of the microprocessor, which is the nucleus of the computer system, is made even higher and the function thereof tends to be further enhanced.
However, the peripheral LSIs have been developed after the microprocessor was developed and, because there are many kinds of LSIs, it takes a longer time to develop the LSIs in general. Therefore, the operation rate of the system bus connected to the peripheral LSI has not been significantly improved. That is, in many cases, the operation rate of the system bus remains lower than that of the high-rate microprocessor so that the former cannot accommodate the high-rate of operation of the latter. Therefore, in the conventional computer system, the frequency of the operation clock of the microprocessor tends to be set to an integer multiple of the frequency of the operation clock of the system bus, in order to easily synchronize the operation clock of the system bus with that of the microprocessor.
Further, in order to inherit the property of software, the newly developed computer systems are required to be compatible with the conventional computer systems. Some software is designed with respect to hardware which is connected to a system bus which operates in response to the operation clock of a relatively low frequency. Therefore, in a case where such hardware is used in the computer system, it is necessary to effect data transfer when the operation rate of the microprocessor is set to correspond to that of the hardware. In this case, the function of the high-speed microprocessor cannot be fully utilized.